The flight capability of a helicopter is provided largely by a main rotor, mounted onto a main rotor shaft. The main rotor shaft is rotated by the helicopter engine, via a main transmission assembly. The main transmission assembly cover, or top case assembly, defines an aperture through which the main shaft extends and onto which is mounted a main bearing. In a Bell 206 helicopter, for example, the main transmission assembly reduces the rate of revolutions of the engine, of about 6000 r.p.m. to the desired rotation rate of about 400 r.p.m. of the main shaft.
The main bearing and the transmission assembly are lubricated by a fluid which flows therethrough and which may be replenished via a filler neck assembly in which there is provided a removable filler cap. When the main rotor is in operation, the lubricant fluid flows in an outward and downward direction through the main transmission assembly, prior to passing, under centrifugal influence, inwards and upwards along the main shaft until it reaches the main bearing and outwards again, via a collector, and once again down towards the main transmission assembly.
In the main transmission assembly, wherein metal components, such as gear wheels, are continually meshing or otherwise making contact with each other, metal fragments tend to be chipped from the components. The chips further abrade the components thus creating yet more chips and so cause further deterioration of the transmission. Periodically, therefore, it may be necessary to replace worn components so as to facilitate continued efficient functioning of the transmission.
Furthermore, accumulation of metal chips could lead to clogging of lubricant passages within the transmission and lead to a reduced cooling thereof by the lubricant. In an extreme case, a continued accumulation of metal chips within the transmission could lead to a complete breakdown while the helicopter is in flight. It is, therefore, known to provide a chip detector in the top case assembly.
A conventional chip detector comprises a magnet and an electrode, electrically insulated from each other, mounted in the top case assembly so as to be exposed to the lubricant fluid flow. The provision of the chip detector within the top cover ensures that any chip will at some stage, be carried into association with the detector, due to the lubricant flow path described above.
As a metal chip in suspension in the fluid is attracted to the detector and contacts it, as it makes electrical contact with the electrode, a short circuit occurs. This facilitates the sending of a warning signal to the pilot.
Chip detectors are conventionally installed, for example, in the Bell 206 helicopter, in the top cover, which forms a main component of the top case assembly. This is achieved by modifying a top cover not originally having a chip detector so as to be able to receive a chip detector in a permanent mounting in contact with the lubricant flow.
It has been found that modification of the top cover as outlined above is very expensive and that it is not necessary to modify the top cover in order to provide an efficient, relatively low cost, chip detector mounting as required in helicopters.